This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus to control gas turbine engine rotor assembly tip clearances during rotor assembly operation.
Gas turbine engines typically include an engine casing that extends circumferentially around a compressor, and a turbine including a rotor assembly and a stator assembly. The rotor assembly includes at least one row of rotating blades that extend radially outward from a blade root to a blade tip. A radial tip clearance is defined between the rotating blade tips and a shroud attached to the engine casing.
During engine operation, the thermal environment in the engine varies and may cause thermal expansion or contraction of the rotor and stator assemblies. Such thermal growth or contraction may not occur uniformly in magnitude or rate. As a result, inadvertent rubbing between the rotor blade tips and the casing may occur or the radial clearances may be more open than the design intent. Continued rubbing between the rotor blade tips and engine casing may lead to premature failure of the rotor blade or larger clearances at other operating conditions which can result in loss of engine performance.
To facilitate optimizing engine performance and to minimize inadvertent rubbing between the rotor blade tips and an inner surface of the shroud, at least some known engines include a clearance control system. The clearance control system channels cooling air to the engine casing to facilitate controlling thermal growth of the engine casing and to thus, facilitate minimizing inadvertent blade tip rubbing. Such cooling air may be channeled from a fan assembly, a booster, or from compressor bleed air sources to impinge on the casing. The effectiveness of the clearance control system may be dependent upon the heat transfer coefficient of clearance control system components.